This invention relates to a bistable switching circuit and particularly to a switching circuit of the kind that is responsive to a low level differential input.
Current Schmitt triggers are known which are used in combination with a differential input transistor pair. Examples of such circuits are described in U.S. Pat. No. 3,805,093 to Hodemaekers and U.S. Pat. No. 3,848,139 to Holt, Jr. However, neither of the patents disclose circuits which assure that no output current is present before regenerative transistor action occurs. This feature is a necessary requirement in some timers where the presence of negative feedback just prior to switching can prevent proper timer operation.
A threshold detector circuit employing cross coupled transistor pairs is disclosed in U.S. Pat. No. 3,514,633 to E. N. Schroeder. A bistable circuit employing a differential input transistor pair and complementary cross-coupled load transistors is disclosed in Japanese Patent Publication No. 53-108363 to Hiroshi Tamura, published Sept. 21, 1978.